KR101293931B1 - Cathode Active Material and Lithium Secondary Battery Comprising the Same - Google Patents

Abstract

The present invention is a cathode active material for a secondary battery, comprising one or more compounds selected from Formula 1, one or more compounds selected from Formula 2, and a combination of one or more compounds selected from Formula 3 Provided is a cathode active material for a secondary battery.
(1-st) [Li (Li _a Mn _(1-axy) Ni _x Co _y ) O ₂ ] * s [Li ₂ CO ₃ ] * t [LiOH]
Li (Li _b Mn _(2-b) ) O ₄ (2)
(1-u) LiFePO ₄ * uC (3)
Wherein 0 <a <0.3, 0 <x <0.8, 0 <y <0.6, 0 <s <0.05, 0 <t <0.05, 0 <b <0.3, 0.01 <u <0.1, and a, b, x, y are molar ratios, and s, t and u are weight ratios.
Such a positive electrode active material provides a high output lithium secondary battery of a non-aqueous electrolyte which has long life and storage characteristics at room temperature and high temperature, exhibits excellent safety as well as excellent rate characteristics and power characteristics.

Description

[0001] The present invention relates to a cathode active material and a lithium secondary battery using the same,

The present invention relates to a positive electrode active material for a secondary battery, comprising a combination of three kinds of compounds, each having a long life and storage characteristics at room temperature and high temperature, and exhibiting excellent safety as well as excellent rate and power characteristics. .

As technology development and demand for mobile devices have increased, there has been a rapid increase in demand for secondary batteries as energy sources. Among such secondary batteries, lithium secondary batteries, which exhibit high energy density and operational potential, long cycle life, Batteries have been commercialized and widely used.

In recent years, there has been a growing interest in environmental issues, and as a result, electric vehicles (EVs) and hybrid electric vehicles (HEVs), which can replace fossil-fueled vehicles such as gasoline vehicles and diesel vehicles, And the like. Although nickel-metal hydride (Ni-MH) secondary batteries are mainly used as power sources of such electric vehicles (EVs) and hybrid electric vehicles (HEVs), lithium secondary batteries of high energy density, high discharge voltage and output stability are used. Research is actively underway and is in some stages of commercialization.

In particular, lithium secondary batteries used in electric vehicles have high energy density and high power output in a short time, and should be able to be used for 10 years or more under severe conditions. Lifespan characteristics are inevitably required.

Lithium ion secondary batteries used in conventional small batteries generally use a layered structure of lithium cobalt composite oxide for the positive electrode and a graphite-based material for the negative electrode, but in the case of lithium cobalt composite oxide, cobalt is a main component Has the disadvantage of being very expensive and not suitable for electric vehicles in terms of safety. Therefore, a lithium manganese composite oxide having a spinel structure composed of manganese having low cost and excellent safety may be suitable as a positive electrode of a lithium ion battery for an electric vehicle.

However, in the case of lithium manganese composite oxide, manganese is eluted into the electrolyte due to the influence of the electrolyte when stored at a high temperature, and thus deteriorates the battery characteristics, and thus an improvement for preventing the manganese compound is required. In addition, since the capacity per unit weight is smaller than that of the conventional lithium-cobalt composite oxide or lithium-nickel composite oxide, there is a limitation in increasing the capacity per cell weight, As shown in Fig.

In order to compensate for each of these disadvantages, a study for producing an electrode with a mixed cathode active material has been attempted. For example, Japanese Patent Application Publication Nos. 2002-110253 and 2004-134245 disclose a technique in which lithium manganese composite oxides and lithium nickel cobalt manganese composite oxides are mixed and used to increase regenerative output. There are still disadvantages of poor cycle life of lithium manganese oxide and limitations in improving safety.

In addition, in order to improve the capacity characteristics, lifespan characteristics and high rate discharge characteristics of the battery, Korean Patent No. 0570417 uses spinel structure lithium dimanganese oxide as a positive electrode active material, and Japanese Patent Application Laid-Open No. 2002-0080448 A positive electrode active material containing a lithium manganese composite oxide was used. In Japanese Patent Laid-Open No. 2004-134245, a secondary battery was manufactured using a positive electrode active material containing a spinel structure lithium manganese composite oxide and a lithium transition metal composite oxide. .

In addition, secondary batteries used in electric vehicles (EVs) and hybrid electric vehicles (HEVs) require excellent rate characteristics and power characteristics according to vehicle operating conditions.

However, in the prior arts known so far, the construction of a secondary battery having a desired level of life characteristics and safety has not yet been proposed.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

After extensive research and various experiments, the inventors of the present application have a combination of three kinds of specific compounds such as a compound of Formula 1, a compound of Formula 2, and a compound of Formula 3 as a cathode active material for a secondary battery, as described later. In addition to the development of a positive electrode active material for a secondary battery, when making a secondary battery using such an active material not only contributes to improving the safety of the battery, but also improves the performance characteristics such as life characteristics, it is excellent in the rate characteristics and power characteristics. It confirmed and completed this invention.

Therefore, the cathode active material for a secondary battery according to the present invention for achieving the above object, one or more compounds selected from the following formula (1), one or more compounds selected from the formula (2), and one selected from the following formula (3) Or a combination of more compounds:

(1-st) [Li (Li _a Mn _(1-axy) Ni _x Co _y ) O ₂ ] * s [Li ₂ CO ₃ ] * t [LiOH]

Li (Li _b Mn _(2-b) ) O ₄ (2)

(1-u) LiFePO ₄ * uC (3)

Wherein 0 <a <0.3; 0 <x <0.8; 0 <y <0.6; 0 <s <0.05 and 0 <t <0.05, 0 <b <0.3 and 0.01 <u <0.1.

Here, a, b, x and y are molar ratios, and s, t and u are weight ratios.

As described above, the present invention is a positive electrode active material by mixing a lithium nickel-manganese-cobalt composite oxide, a spinel structure lithium manganese oxide, and an olivine structure lithium iron phosphate having a layered structure and a specific element and compound composition It is characterized by using.

Although the specific elemental composition and the action of lithium carbonate (Li ₂ CO ₃ ) and lithium hydroxide (LiOH) in the compound of formula (1) cannot be clearly explained, the compound of formula (1) together with the compound of formula (2) and compound of formula (3) When used, it is assumed to act to compensate for each drawback and maximize the advantages. This will be described in detail as follows.

Compounds of formula (2) and compound of formula (3) have the advantage of excellent thermal safety and low cost, but has a problem of small capacity and poor high temperature characteristics. In particular, the compound of Formula 2 easily evaporates Mn due to structural problems such as Jahn-teller distortion, which is more clearly indicated by the HF present inside the cell.

On the other hand, in the positive electrode active material of the present invention, the compound of formula (1) having excellent structural stability and basicity, the compound of formula (2) and / or the compound of formula (3) and / or the inside of the battery having excellent safety but structural instability in high temperature storage, etc. By suppressing the reaction of HF, a strong acid present, and inducing HF toward the compound of Formula 1, the amount of Mn elution of the compound of Formula 2 can be minimized and degradation of the compound of Formula 3 can be suppressed. In addition, stability and the like, which are inherent properties of the compound of formula 3, can also be expressed thereby.

Lithium carbonate (Li ₂ CO ₃ ) and lithium hydroxide (LiOH) in the formula (1) contains a predetermined amount, preferably less than 0.05% by weight relative to the entire compound of Formula 1, more preferably may be less than 0.03 weight ratio. If the content of lithium carbonate (Li ₂ CO ₃ ) and lithium hydroxide (LiOH) is 0.05 weight ratio or more, it is not preferable because the battery capacity may be reduced and side reactions may be caused.

It is preferable that the compound of Formula 1 is included in a ratio of 10 to 90% by weight based on the total weight. If the content of the compound of formula 1 is too small in the compound, the desired capacity cannot be obtained. On the contrary, if the content of the compound of formula 1 is too large, the safety of the battery is deteriorated. . For the same reason as above, the compound of Formula 1 is more preferably contained in a proportion of 50 to 90% by weight based on the total weight.

The compound of formula 2 is included in the ratio of 5 to 80% by weight based on the total weight, the compound of formula 3 is preferably included in the ratio of 10 to 70% by weight based on the total weight. Too little compound of the formula (2) is not expected to improve the safety, on the contrary too large is not preferable because the capacity is reduced. In addition, too little compound of the formula (3) is difficult to expect the improvement of stability, on the contrary too much is not preferable because there is a difficulty in the process and the energy density is reduced. For this reason, a more preferable content range is 5 to 50% by weight for the compound of Formula 2, and 10 to 30% by weight for the compound of Formula 3.

When the content of the compounds of Formulas 2 and 3 is too small, a problem of deterioration of battery safety may occur. On the contrary, when the content of the compounds of Formulas 2 and 3 is too large, a desired lifespan characteristic may not be obtained. Not desirable

Mn in the compound of Formula 2 may be substituted with other elements that may be located in the six coordination structure, for example, Al, Mg and the like. The substitution amount of such a substitution element is preferably 0.2 mole ratio or less relative to the total transition metal, and when the substitution amount is too large, it is difficult to secure a desired level of capacity.

In addition, in the compound of Formula 2, oxygen (O) ions may be substituted with other anions in a predetermined amount range. Such substituted anions may be one or two or more elements preferably selected from the group consisting of F, Cl, Br, I, S, chalcogenide compounds, and nitrogen.

By the substitution of these anions, the bonding strength with the transition metal is excellent and the structural transition of the positive electrode active material is prevented, thereby improving the life of the battery. On the other hand, if the amount of substitution of the anion is too large (greater than 0.5 molar ratio relative to the total anion), it is not preferable because the compound of Formula 2 does not maintain a stable spinel structure and thus the lifespan characteristics are lowered. Therefore, the substitution amount of a preferable anion is 0.01-0.5 molar ratio with respect to all anions, and more preferable substitution amount of an anion is the range of 0.01-0.2 molar ratio. The total anion is a concept including both oxygen and a substituted anion.

On the other hand, in the case of the olivine-type lithium iron phosphate, such as the formula (3), since the olivine-type lithium iron phosphate itself is very low conductivity, the form of the formula (3) containing a good conductivity carbon is preferred. More preferably, the carbon is included in a form coated on the surface of the olivine-type lithium iron phosphate.

In one preferred embodiment, among the compounds of the formula (3), a material that is stable in the coordination structure to the phosphorus (P) site may be doped. The material that is stable in the four coordination structure may be one or two or more elements selected from the group consisting of Si, V and Ti. Preferable doping amount is 0.01-0.4 molar ratio, More preferable substitution amount is the range which is 0.01-0.1 molar ratio.

As in the compound of Formula 2, the compound of Formula 3 may also be substituted with other anions in oxygen (O) ions in a predetermined range, in which case the substituted anions are preferably F, Cl, Br, I, It may be one or two or more elements selected from the group consisting of S, chalcogenide compounds, and nitrogen. The preferred amount of substitution is 0.01 to 0.4 moles relative to the total anion, and more preferably the amount of substitution may range from 0.01 to 0.1 moles.

Since the compound of Formula 1, the compound of Formula 2, and the compound of Formula 3 constituting the cathode active material of the present invention may be prepared based on the composition formula, description of such a manufacturing method is omitted herein.

This invention also provides the mixture for positive electrodes containing the positive electrode active material mentioned above, a electrically conductive agent, and a binder.

The mixture may contain a predetermined solvent such as water or NMP to form a slurry. The slurry may be coated on the positive electrode current collector, followed by drying and rolling to produce the positive electrode.

The present invention provides a positive electrode for a secondary battery comprising the mixture for the positive electrode.

The positive electrode is produced by, for example, applying a slur made of a mixture of the positive electrode active material, the conductive agent and the binder according to the present invention onto a positive electrode current collector, followed by drying, and, if necessary, the positive electrode active material and the conductive material. At least one material selected from the group consisting of a viscosity modifier and a filler may be further included in a mixture (electrode mixture) such as a binder.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The conductive material is a component for further improving the conductivity of the electrode active material, and may be added at 0.01 to 30 wt% based on the total weight of the electrode mixture. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery, for example, graphite such as natural graphite or artificial graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Carbon derivatives such as carbon nanotubes and fullerenes, conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component that assists in bonding of the active material and the conductive agent to the binder and in binding to the current collector, and is usually added in an amount of 1 to 50% by weight based on the total weight of the mixture containing the cathode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose, polyvinylidene fluoride and the like, but are not limited thereto. In some cases, the above-described solvent may play a role as a viscosity adjusting agent.

The filler is optionally used as an auxiliary component for suppressing the expansion of the electrode. The filler is not particularly limited as long as it is a fibrous material without causing chemical change in the battery, and examples thereof include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

In addition, the present invention provides a lithium secondary battery composed of the positive electrode, the negative electrode, the separator and a lithium salt-containing non-aqueous electrolyte.

The negative electrode is manufactured by coating and drying a negative electrode material on a negative electrode current collector, and if necessary, components such as a conductive agent and a binder as described above may be further included.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti which can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

The separation membrane is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the membrane is generally 0.01 ~ 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. Such separation membranes include, for example, olefinic polymers such as polypropylene, which are chemically resistant and hydrophobic; A sheet or nonwoven fabric made of glass fiber, polyethylene or the like is used. When a solid electrolyte such as a polymer is used as an electrolyte, the solid electrolyte may also serve as a separation membrane.

The lithium salt-containing non-aqueous electrolyte is composed of a non-aqueous electrolyte and a lithium salt. As the nonaqueous electrolyte, a nonaqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte, and the like are used.

Examples of the nonaqueous electrolytic solution include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, But are not limited to, lactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, Nitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives , Tetrahydrofuran derivatives, ether, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, Polymers containing ionic dissociation groups, and the like can be used.

As the inorganic solid electrolyte, for example, Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates, and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

For the purpose of improving charge / discharge characteristics, flame retardancy, etc., non-aqueous electrolytes may be used in the form of, for example, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. are added It is possible. In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene carbonate, PRS (propene sultone), FEC (fluoro-ethylene carbonate), and the like.

In particular, the secondary battery according to the present invention may be used as a unit cell of a battery module which is a power source for medium and large devices requiring high temperature safety, long cycle characteristics, high rate characteristics, and the like.

Preferably, the medium-to-large device may be an electric vehicle (EV), a hybrid electric vehicle (HEV), or a plug-in hybrid electric vehicle (HEV).

As described above, the nonaqueous electrolyte secondary battery using a mixture of a specific lithium nickel-manganese-cobalt composite oxide, a lithium manganese oxide, and a specific lithium iron phosphate as a cathode active material according to the present invention can ensure safety and at high temperature conditions. Not only can the life be improved, but excellent rate characteristics and power characteristics can be obtained.

Hereinafter, the content of the present invention will be described in detail by way of examples, but the scope of the present invention is not limited thereto.

&Lt; Example 1 >

_{0.9967Li (Li 0 .1 (Ni 0} .5 Mn 0 .4 Co 0 .1) 0.9) O 2 * 0.0021LiOH * 0.0012Li 2 CO 3 and 0.98 (LiFePO ₄₎ * 0.02C and Li (Li _{0 .1 Mn 1 .8 Al 0 .1) O} 4 for (70: 30) * 0.9: 10 (ratio by weight) of the conductive active material and a binder 90 including a ratio of 4 (active material: conductive material 6: the binder, the weight ratio ), And coated on Al-foil so that the loading amount of the final electrode was 1.6 mAh / cm ² . Were negative by the use of carbon, the electrolyte is EC: used was prepared by dissolving LiPF ₆ to be in a 1M 2: EMC = 1. Based on this, a pouch-shaped monocell was constructed.

<Example 2>

A pouch-type monocell was constructed in the same manner as in Example 1 except that the ratio of the active material was set to (70:30) * 0.95: 5 (weight ratio).

&Lt; Comparative Example 1 &

To 30 (weight _{ratio): 0.9967Li (Li 0 .1 (} Ni 0 .5 Mn 0 .4 Co 0 .1) 0.9) O 2 * 0.0021LiOH * 0.0012Li 2 CO 3 and 0.98 (LiFePO ₄₎ 70 to 0.02C A pouch-type monocell was constructed in the same manner as in Example 1 except that a positive electrode active material was included.

<Experimental Example 1>

The batteries prepared in Examples 1 and 2 and Comparative Example 1, respectively, were discharged at 120 mA at 25% state of charge (SOC), 50% SCO, and 75% SOC for 10 seconds to measure resistance and power. . The results are shown in Table 1 below.

[Table 1]

In the case of an electrode in which an olivine-structured material is mixed with a layered material, it is possible to improve safety and lifespan. If additional agents are added, there is a problem that the capacity is reduced.

On the other hand, as shown in Examples 1 and 2 of Table 1, when using three specific materials at the same time, it is possible to obtain a satisfactory level of power while also maximizing stability and safety. Although not clearly identified, it is presumed to be the influence of a third manganese spinel-based material having high density and high electrical conductivity. Particularly, in the case of the layered active material of the present invention, by simultaneously containing LiOH and Li ₂ CO ₃ , it is possible to exhibit excellent properties of reducing Mn dissolution through the reaction with HF known to occur in the manganese spinel active material.

Those skilled in the art to which the present invention pertains will be able to make various applications and modifications within the scope of the present invention based on the above contents.

Claims (8)

As a positive electrode active material for a secondary battery, characterized in that it comprises one or two or more compounds selected from Formula 1, one or two or more compounds selected from Formula 2, and a combination of one or two or more compounds selected from Formula 3 Secondary battery positive electrode active material:
(1-st) [Li (LiaMn _(1-axy) Ni _x Co _y ) O ₂ ] * s [Li ₂ CO ₃ ] * t [LiOH] (1)
Li (Li _b Mn _(2-b) ) O ₄ (2)
(1-u) LiFePO ₄ * uC (3)
Wherein 0 <a <0.3, 0 <x <0.8, 0 <y <0.6, 0 <s <0.05, 0 <t <0.05, 0 <b <0.3, 0.01 <u <0.1, and a, b, x, y are molar ratios, and s, t and u are weight ratios. The cathode active material of claim 1, wherein the compound of Formula 1 is included in a proportion of 20 to 90 wt% based on the total weight. According to claim 1, wherein the compound of Formula 2 is contained in a ratio of 5 to 80% by weight based on the total weight, the compound of Formula 3 is contained in a ratio of 10 to 70% by weight based on the total weight A secondary battery positive electrode active material characterized in that. A cathode mixture for secondary batteries, comprising the cathode active material according to any one of claims 1 to 3. A positive electrode mixture according to claim 4 is coated on a current collector. A lithium secondary battery comprising the positive electrode according to claim 5. The lithium secondary battery according to claim 6, wherein the lithium secondary battery is used as a unit battery of a battery module that is a power source of a middle- or large-sized device. 8. The lithium secondary battery of claim 7, wherein the medium-to-large device is an electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle.